At 12:19 AM -0600 12/24/07, Galchin Vasili wrote:
module Main where import Control.Concurrent.STM import Control.Concurrent import System.Random
type Account = TVar Int
transfer :: Account -> Account -> Int -> IO () transfer from to amount = atomically (do {deposit to amount; withdraw from amount})
deposit :: Account -> Int -> STM () deposit acc amount = withdraw acc (- amount)
withdraw :: Account -> Int -> STM () withdraw acc amount = do { bal <- readTVar acc; writeTVar acc (bal - amount) }
When I try
withdraw (TVar 10) ..
I get "Not in scope: data constructor 'TVar'. OK .. "Tvar" is a class and not a Haskell data type ... so now what?
You first need to create an account:
acct <- newTVar 100
Then you can withdraw from it:
atomically (withdraw acct 10)
Dean